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Definition:
The relative amounts of polymers of different molecular weights that comprise a given specimen of a polymer. Two samples of the same polymer with the same weight-average may perform quite differently in processing because they have different molecular weight distributions.

The ratio of the weight-average molecular weight to the number-average molecular weight gives an indication of the distribution. Two basic groups of methods are used for measuring molecular weight distributions: (1) Fractionation methods, which actually divide the specimen into fractions of various molecular weight ranges, include fraction precipitation and fraction solution (these two are the most widely used), chromatography, liquid-liquid partition, ultracentrifugation, zone refining, and thermogravimetric diffusion. After fractionation by any of these methods, the weight of each fraction is plotted against the average molecular weight of each fraction to obtain a curve of the distribution. (2) Methods for estimating molecular weight distribution without fractionation include light scattering studies, electron microscopy, dilute solution viscosity measurements, gel permeation chromatography, ultracentrifugation and diffusion.